1. Field of the Invention
The present invention relates to an optical waveguide, and more particularly to an optical waveguide transmitting light wave energy in a single mode for optical communication made of transparent dielectric materials such as glass.
2. Description of the Prior Art
The important things for an optical waveguide for optical communication are that the reduction of light energy in transmission, i.e. the transmission loss of light energy is low, it has a wide band signal frequency response, and the launching of light in the waveguide, i.e. the connection between a light source and a waveguide or the connection between waveguides is easy.
To date various ones have been proposed as the optical waveguide among which a singly cladded optical fiber is known as an effective one as for optical communication. This optical fiber is composed of a transparent central dielectric body, i.e. a core having a high refractive index and a covering layer, i.e. a cladding consisting of a material having a lower refractive index.
The optical characteristic of this singly cladded optical fiber is determined mainly by the parameter expressed by the formula ##EQU1## WHERE A IS THE RADIUS OF THE CORE, .lambda. IS THE WAVELENGTH OF LIGHT, N.sub.1 AND N.sub.2 ARE THE REFRACTIVE INDICES OF THE CORE AND THE CLADDING, RESPECTIVELY, AND THE PARAMETER V WILL HEREINAFTER BE REFERRED TO MAINLY AS THE NORMALIZED FREQUENCY THOUGH ORDINARILY IT IS CALLED IN VARIOUS WAYS.
In the range of the value of v no larger than 2.4083 the propagation mode of light in the optical waveguide is single, while a plurality of propagation modes occur in the range of v higher than that value.
For the optical waveguide with the value of v higher than 2.4083 the band width of signal frequency is narrowed by the difference in the group delay for modes.
Consequently, for the optical waveguide for optical communication requiring a wide band the value of v has to be designed so that light is propagated in a single mode. To this end the radius (a) of the core or the difference between n.sub.1 and n.sub.2 has to be made small. However, there is a certain limitation to making the difference between n.sub.1 and n.sub.2 small from the problem of manufacture or the bending loss. Consequently, the radius of the core has been very small. This makes it very difficult to connect a light source with an optical waveguide or to interconnect optical waveguides.
Further, even in an optical waveguide transmitting light in a single mode, when a light signal has a wide spectral width, a group delay difference occurs in the spectral region to narrow the signal frequency band width.